Directionality of sound

[Music_Smurf]

From my understanding, high frequency sound waves are very directional, and low frequency sound waves are nondirectional. At what frequency do sound waves become nondirectional?

In my home theater system, I have the fronts, center, and surrounds set to "small," and I have the subwoofer crossed over at 80Hz. The subwoofer is located in the corner. When watching movies, I can tell there is low frequency sound coming from the corner instead of being nondirectional. It is very irritating. It is no fun watching a movie when the sound placement does not coincide with the action on the screen.

[HDBRbuilder]

To understand soundwaves it is best to consider their graphic representation as on an audio oscilloscope. In this representation, any constant particular frequency is represented by a continuous line that looks like an undulating line...going upwards toward its peak, whereupon it curves over and continues downward toward its trough, again arcing upwards and continuing the sequence, ad infinitum. In this representation, the distance between two of its peaks OR two of its troughs is a cycle. Frequencies are measured in cycles per second, so a 32 hz frequency completes 32 cycles in a second, whereas a 32,000 hz frequency completes 32,000 cycles in the same second. High frequencies complete more of these cycles in a second than low ones do. The higher the number of cycles a frequency produces, then the tighter (or "narrower")the band of it...peaks/troughs are closer together...the opposite is true for the lower frequencies.

When a speaker reproduces these sound waves, it has to move more air for the lower frequency ones than for the higher ones, and the air is moved more slowly for the lower ones than for the higher ones, too, because their cycles are fewer per second. Because less air has to be moved for the higher frequencies, and the air is moved so many more times per second for them, they tend to go out in a straighter line, than the lower ones do, while the lower ones tend to radiate out farther off the centerline axis. Therefore, unless the centerline of the speaker is pointed right at one's ears (or very close to them) on this axis, the high frequencies are not gonna be DIRECTLY picked up by them....but on the other hand, you can walk around the room and pick up the lower frequencies just fine...since they have less "directivity"!

Keep in mind that our notions of "directivity" are based upon how we "hear" sound to begin with!!

Your body uses much more than just your ears to "hear" with. Just consider your ear is like a speaker...with the part that sticks out from your head being like the cone, and your eardrum being like the voice-coil. Not a very big speaker is it?

Have you ever plugged a speaker or headphones into a microphone jack?...then talk into it and listen to the sound coming out from another speaker? Try it some time as an experiment...just consider your ear as a small speaker that is being used like a microphone to pick up sound....with the part that sticks out from your head being the cone(or horn lens). In the experiment I mentioned, the smaller the speaker used as a microphone, then the less amount of LOW frequencies it can pick up EFFICIENTLY and transfer through the amp to the speaker you are listening to...the larger the speaker you use as a microphone, the less high frequencies it picks up EFFICIENTLY and will come out of the other speaker.

Now we all know that we don't have 15" driver sized ears...or at least MOST of us don't...LOL! So in order for us to hear those low frequencies, something besides just our ears HAS to send it to our brain...and that is our body's mass.

Low frequencies are transferred through our body's mass to the parts of our middle ear, and are in turn transferred to our brain as a signal...because our little eardrum "voice coil" just can't pick it up well.

Now even though our body acts as a monaural microphone in this manner, we are still able to determine the direction from which this low frequency pick-up is coming because we "feel" the direction on the side of our body that picks it up the most! If these lower frequencies are being carried to us through the floor, as in how the lowest ones are normally picked up by us, then our feet feel it when they are in contact with the floor...if it is a kick drum right in front of us slamming its sound wave into our body, then that part feels it....and we identify its direction from our sense of feel in our skin.

There is a big hoopla on positioning of subwoofers, and depending on exactly how low those subs begin to kick in, determines just whether they are gonna be picked up only by our body mass, or whether our actual ears will pick up some of it...in some cases, only one sub is needed, because only our body mass picks up its frequencies.

Now...because it is actually our EARS that pick up the higher frequencies, then the more those frequencies are DIRECTED towards our ears, then the better they pick them up!

Ears are more like a midrange driver, working in reverse as a microphone...and that is why they pick up those frequencies the most efficiently.

When PWK said,"the midrange is where we live", that is what he meant.

All of this is kinda a way to just say: There is no magic point at which sound waves become non-directional...how we each hear them, and the source we hear them from, determines how directional they are...and each of us hears them differently to some extent! But generally speaking, the higher up the frequency range, the more directional they become...to our ears!

Your EARS are still doing some of the "hearing" for you at 80 Hz, and that is why the frequencies in that area from that sub are still "directional" to you.

[jdm56]

80-100hz is the area where most people agree that woofers (and subs) tend to become non-directional. If your low pass is set in that range, and you still can localize the sub, perhaps the crossover slope is not steep enough. Or the bass level may be too high.

[Colin]

Placement of the sub woofer is one of the areas which audiophile debate. The theory is that subwoofers should go in the corner for the lowest possible response, which is true. The practice however, is that they are best located at the apex of the curve of the soundstage, which is also true. If you hear the lowest note on the bass guitar, 40-Hz, can you tell where it came from? The answer is yes. We do know where the bass player is standing.

How so? Because the lowest note on a bass guitar is not a single isolated peak on the notes wave form. There are other peaks and valleys, which form the harmonics of the note. These peaks and valleys ascend and descend on both directions away from the initial 40-Hz pulse, like wiggles in a rope. Even a 40-Hz note has harmonics which ascend all the way up the audible scale. It is the harmonics which identify the note as a bass guitar sound. Even if the initial bass peak is not easily locatable, the additional upper harmonics are. Therefore, single subs can often sound better when located at the middle of the soundstage, while dual subs are best placed at both ends.

You may be using too much volume on the sub (quite common, unless you love live acoustic music and are used to naturally balanced sound) or you may have too much equalization (again quite common if you are trying sock and punch in your movies). But try locating your sub near the center of your stereo and see if that doesnt make the sound gel like Jello in the fridge.